1. Field of the Invention
The present invention relates to a light receiving element and a semiconductor laser device, and more particularly to a configuration of a light receiving element utilized for a pickup or the like of an optical disk and to a semiconductor laser device including the light receiving element.
2. Description of the Background Art
A semiconductor laser device utilized for a pickup of an optical disk has conventionally been known. An example of such a semiconductor laser device is shown in FIG. 5.
The semiconductor laser device shown in FIG. 5 is disclosed in Japanese Patent Laying-Open No. 7-142813, and includes a stem platform 9 and a stem 11 fixed to stem platform 9. A semiconductor laser chip 17 is attached to the side surface of stem 11, a photodiode chip for detection is attached to the top surface, and a photodiode (light receiving element) 16 is attached onto stem platform 9.
A cap 12 having a window 13 is fixed to stem platform 9. Glass 10 is mounted on cap 12 and fixed to cap 12 by bonding resin 14.
A vent hole 15 is provided on stem platform 9, which allows emission of moisture even when the moisture is externally introduced into a space within cap 12 through bonding resin 14.
However, the semiconductor laser device has a problem such that moisture entered into the space described above from vent hole 15 corrodes a metal portion unprotected by a surface protection film, such as a bonding pad portion of photodiode (light receiving element) 16.
An example of a light receiving element capable of addressing such a problem is a light receiving element disclosed in Japanese Patent Laying-Open No. 9-97892. The light receiving element described in this publication is shown in FIG. 6.
Referring to FIG. 6, the light receiving element includes a bonding pad portion 20, a photodiode portion 21 and a signal processing circuit portion 22. Bonding pad portion 20 electrically connects, for example, photodiode portion 21 and signal processing circuit portion 22, photodiode portion 21 detecting received light, and signal processing circuit portion 22 processing a light signal.
The light receiving element further includes a semiconductor substrate 1, and an oxide film 2 formed thereon. Semiconductor substrate 1 includes a p-type semiconductor substrate 1a and an n-type epitaxial layer 1b. Various impurity diffusion layers are formed, and a metal layer (an electrode) connected to a predetermined impurity diffusion layer is also formed within semiconductor substrate 1.
Bonding pad portion 20 is constituted by a part of a metal layer 4a formed on oxide film 2, a part of the surface of bonding pad portion 20 not being covered by a surface protection film 5. A metal layer having corrosion resistance such as TiW layer 7 and Au layer 8 is formed on the part of the surface. Au layer 8 is formed in order to enhance the bonding property.
By forming the metal layer having corrosion resistance such as TiW layer 7 on bonding pad portion 20 as described above, corrosion of bonding pad portion 20 can be inhibited even if moisture enters into the space within cap 12 from vent hole 15 as described earlier.
However, the conventional example shown in FIG. 6 also has a problem described below. This problem is described with reference to FIG. 7. FIG. 7 is an enlarged view of bonding pad portion 20 and portions adjacent thereto.
Referring to FIG. 7, a bonding wire 6 is formed on bonding pad portion 20. If there is a projection on, for example, the surface of metal layer 4a, TiW layer 7 or Au layer 8 is damaged at the time of bonding of bonding wire 6, so that metal layer 4a located underneath TiW layer 7 is corroded, and thus a corrosion region 18 is locally generated as shown in FIG. 7. Corrosion region 18 is generated mainly at the periphery of bonding wire 6, and if corrosion region 18 reaches interconnection portion 23, a problem may be raised such that bonding wire 6 and an interconnection portion 23 are not electrically conducted. This problem may also occur if TiW layer 7 or the like is damaged by contacting a probe with TiW layer 7 in a product test.
The present invention is directed to solve the problems described above. It is an object of the present invention to enhance corrosion resistance of a light receiving element.
According to one aspect of the present invention, a light receiving element includes a bonding pad portion, an interconnection portion extending from the bonding pad portion, a corrosion-resistant conductive layer formed of a corrosion-resistant material and extending from the bonding pad portion to the interconnection portion, and a bonding wire formed on the bonding pad portion. The corrosion-resistant conductive layer herein typically represents a conductive layer formed of a material having low corrosiveness (hard-to-corrode property) to a corrosive factor such as moisture, compared to materials constituting the bonding pad portion and the interconnection portion.
By extending the corrosion-resistant conductive layer from the bonding pad portion to the interconnection portion as described above, the interconnection portion and the bonding wire can be electrically connected by the corrosion-resistant conductive layer even if the bonding pad portion located on the periphery of the bonding wire is corroded.
The light receiving element according to the present invention includes a protection film having an opening on the bonding pad portion and covering the interconnection portion, the corrosion-resistant conductive layer extending underneath the protection film.
By thus forming the corrosion-resistant conductive layer such that it extends to the underneath the protection film, the bonding pad portion and the interconnection portion can be electrically connected by the corrosion-resistant conductive layer, and hence the effect described above can be obtained.
Further, an additional corrosion-resistant conductive layer may be formed between the bonding pad portion and the bonding wire.
This can inhibit corrosion of the bonding pad portion itself located on the periphery of the bonding wire, and can further ensure connection between the interconnection portion and the bonding wire.
The bonding pad portion and the interconnection portion are formed on an insulating film, and a corrosion-resistant conductive layer is arranged to be in contact with the insulating film.
By thus arranging the corrosion-resistant conductive layer on the insulating film side, the corrosion-resistant conductive layer can be protected by the bonding pad portion. Further, adhesion strength between the insulating film and the corrosion-resistant conductive layer can be maintained even if the bonding pad portion located on the periphery of the bonding wire is corroded. As a result, defections of the bonding pad portion can effectively be inhibited.
According to another aspect of the present invention, a light receiving element includes an Al alloy layer formed on an insulating film, a protection film having an opening on a part of the surface of the Al alloy layer and covering the Al alloy layer, and a bonding wire formed on a part of the surface of the Al alloy layer. The Al alloy layer then includes a TiW layer extending from underneath the opening to underneath the protection film.
By extending the TiW layer, which is an example of a corrosion-resistant conductive layer, from underneath the opening to underneath the protection film as described above, the Al alloy layer located underneath the protection film and the bonding wire can be electrically connected by TiW layer, even if the Al alloy layer located underneath the opening is locally corroded. Further, an Al alloy usable as an interconnection material of a circuit portion in the light receiving element, and TiW usable as a barrier metal in the circuit portion are used, so that no further complicated processes are required. Thus, the element can be offered at a low cost.
The TiW layer is formed underneath the Al alloy layer to be in contact with the insulating film, and further, TiW layer and Au layer are preferably formed between the Al alloy layer and the bonding wire. This can inhibit corrosion of the Al alloy layer located underneath the opening, and can further ensure the electrical connection between the Al alloy layer and the bonding wire.
The light receiving element preferably includes a signal processing circuit. Thus, a light receiving element incorporating the signal processing circuit can be improved in its corrosion resistance.
A semiconductor laser device according to the present invention includes the light receiving element described above. Thus, the semiconductor laser device can also be improved in its corrosion resistance.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.